CCD imagers of the line-transfer type have an image register comprising a plurality of equal-length charge transfer channels in a parallel array into which a radiant energy image is projected. Elements of this image are photoconverted to charge packets accumulating in the charge transfer stages of the ones of these charge transfer channels receiving static clocking signals. During the picture, or active portions of the field trace interval successive ones, successive alternate ones, or successive pairs of the charge transfer channels are selected at line scan rate to receive dynamic clocking signals for at least the line trace portions of the following line interval. Responsive to this dynamic clocking each selected charge transfer channel transfers the charge packets accumulated therein into a CCD line multiplexer and thence to a charge sensing stage to generate a line of video signal samples. The line multiplexer may be a side-loaded CCD shift register, by way of example; or, by way of further example, it may be a charge funnel, a CCD shift register with wide enough input port to span the parallelly aligned output ports of the charge transfer channels in the image register and with a relatively narrow output port to the charge sensing stage.
Transfer smear arises in a semiconductor imager when there is transfer of charge packets past areas of intense radiant energy in the image. Transfer smear has been a problem in CCD imagers of field transfer type, which has been avoided by shuttering these imagers to interrupt image register irradiation during field transfer. Shuttering a CCD imager of line transfer type is impractical to do, as can be readily appreciated by one skilled in optical system design. In the present inventor's previous work, transfer smear in video signals supplied from unshuttered field transfer CCD imagers has been suppressed. In general this is done using an electrical technique wherein a black level signal that is contaminated with transfer smear is differentially combined with the video signal contaminated with transfer smear.
In making provisions for electrically compensating for transfer smear one becomes aware that the problem can be subclassified according to whether the image does or does not change from field to field. Field to field change of all image element occurs, for example, during the panning of a television camera. Panning and many types of image movement across screen involve relative movement of imager and image in predetermined horizontal direction. Transfer smear artifacts have a tendency to be more disturbing to the viewer of a reconstructed television image from a CCD imager output signal when these artifacts appear in a direction perpendicular to motion rather than in the direction of motion. Transfer smear artifacts appear as vertical streaking in a field transfer type of CCD imager, but as horizontal streaking in a line transfer type of CCD imager.
The severity of transfer smear in a CCD imager is related to the time it takes to move a charge packet past an area of intense radiant energy in the image. The speed with which charge can be moved in a CCD device for a given amount of drive power is limited by the RC time constant associated with the resistance of the clock signal source and the capacitance of the gate electrodes being driven respective to elements at static potential. In a field transfer type of CCD imager, where all the charge tranfer channels in the image and field storage registers are simultaneously forward clocked during field transfer, field transfer times typically are of the order of ten scan line times, each scan line time comprising a line trace and a line retrace interval. In a line transfer type of CCD imager forward clocking of a selected charge transfer channel usually takes place in one line trace interval, and the number of charge packets transferred in one line time is two to three times the number of charge packets (i.e., one per field line) transferred during field transfer in a field transfer CCD imager. So transfer smear is one-tenth to one-thirtieth so severe in an unshuttered CCD imager of line transfer type as it is in an unshuttered CCD imager of field transfer type, but transfer smear is nevertheless a significant problem.